Dielectric resonators (DRs) are frequency-determining components in microwave devices which confine electromagnetic energy through total internal reflection by virtue of its high dielectric constant. In order to use a ceramic material for DR application, the material should have high dielectric constant (∈r>10), Qu>2000 and near zero temperature coefficient of resonant frequency (τf). Since the lower end of the microwave frequency spectrum are overcrowded, it is essential to use Ku and millimeter wave frequencies for wireless communications wherein low dielectric frequency determining components are a must.
It is known in the art that low dielectric ceramic materials can be gainfully exploited as particulate fillers for the development of filled fluoropolymer laminates such as PTFE (polytetrafluoroethylene) laminates. Such filled PTFE laminates with relative permitivities/dielectric constant of 2.94±0.04, 6.15±1.5 and 10.2±25 are available commercially with trade name RT/Duroid@ 6002, 6006 and 6010. Most of the low end microwave devices use glass fibre reinforced epoxy (FR-4) based circuit boards. However, these microwave circuit boards suffer from high loss tangent and low dielectric breakdown voltage. Many of the high power microwave devices demand circuit boards with low loss tangent (≦0.002), superior break down voltage threshold together with relative permittivity at par with FR-4 laminates. One of the reasons for the non-availability of equivalent filled PTFE laminates is attributed to the lack of high quality factor and low dielectric (13-15) microwave ceramic as particulate fillers.
Another important problem is the variation of dielectric constant of filled PTFE substrates with respect to temperature. This problem has so far been addressed by making compositions of polymer filled with more than one particulate ceramic material to control the temperature coefficient of dielectric constant. This approach is described in U.S. Pat. No. 5,358,775 where in a high dielectric constant (K≧4), low coefficient of dielectric constant (TCK≦150 ppm/° C.) electrical substrate material comprises a fluoropolymer (preferably PTFE) filled with Class 1 capacitor material such as barium neodymium titanate together with secondary fillers such as silica and alumina. Another important aspect of the aforementioned investigation is that the coefficient of thermal expansion of the composite must also be sufficiently low (CTE≦35 ppm/° C.) to result in composites which consistently maintains a low TCK. The same approach is extended in U.S. Pat. No. 5,552,210, wherein a high dielectric (K≧5) and comparatively low thermal coefficient (TCK<200 ppm/° C.) polymeric composite matrix is disclosed using particulate fillers such as titania, magnesia, alumina etc. incorporated in the fluoropolymeric matrix to tune the dielectric properties. Temperature stable substrate materials have also been realized in the ceramic filled non-fluoropolymeric based systems which is disclosed in U.S. Pat. No. 5,223,568 where in a ceramic filler blend comprising of barium nanotitanate_and silica is incorporated in the poly(1,2-butadiene) liquid resin. High capacitance laminates made of thin films of polytetrafluoroethylene filled with large amounts (25 to 85 vol %) of dielectric filler, in which the films are plated or clad with conductive material are disclosed in U.S. Pat. No. 4,996,097. The high dielectric fillers includes TiO2, BaTiO3 or a Ferro-electric complex.
A polymer composition having high dielectric constant, which varies little with temperature, has been disclosed in U.S. Pat. No. 5,739,193 which is made from poly(phenylene sulfide) (PPS), a thermoplastic polymer, filled with strontium titanate, barium neodymium titanate, barium strontium titanate/magnesium zirconate etc. as primary fillers and mica as the secondary filler material. A polymeric composition which has a dielectric constant K>4 at 20° C. which varies little with temperature is disclosed in U.S. Pat. No. 5,965,273 wherein a composite is made from a polymer or mixture of polymers and a ceramic or a mixture of ceramics where the polymer or mixture of polymer has a dielectric constant K in the range of about 1.5 to about 3.5 and a temperature coefficient of dielectric constant TCK that is negative and is between 0 and about −300 ppm/° C. The polymer is selected from a group consisting of poly(phenylene sulfide), cycloolefinic copolymer and mixtures thereof filled with a first ceramic filler material consisting of calcium zirconate, strontium zirconate, CaTiSiO5, PbZrO3, zirconia and mixtures thereof, each having dielectric constant in the range of about 15 to about 200 and TCK>0 up to about 300 ppm/° C. and an optional second ceramic comprising of one or more ceramic selected from the group consisting of aluminum oxide, magnesium titanate, mica, silicon dioxide, beryllia, spinel and thoria each having a dielectric constant in the range of about 15 and a TCK>0 up to about 300 ppm/° C. However, the use of secondary fillers which is used to control the TCK of the composite system often adversely affect fine control over relative permittivity, homogeneity, rheology etc. of the composite systems. This could be the reason why such temperature stable high dielectric PTFE based thermoplastic composite systems are not available in the open market for commercial use.
In order to overcome the difficulties of the prior art and to provide for superior components such as filled fluoropolymer laminates having enhanced properties, the Applicants herein describe the best possible mode of arriving at such laminates having all the desired properties best suited for microwave devices which will help them perform in a better way. Thus, the present disclosure is able to overcome all the associated drawbacks in this field of microwave technology.